The largely passive Egyptian army defends just the outer perimeter of the prodemocracy demonstrations in Tahrir Square against the gangs of supporters of President Hosni Mubarak who lurk on the periphery of central Cairo. Inside the square, security is provided by a volunteer army. Young men search bags, give a light frisk and ask everyone passing through to hold up their ID cards to ensure that no plainclothes government agents have infiltrated the crowd. They are all polite. "We are very sorry" is a common refrain. "This is for your safety." Many are religious, with thick beards, some with quarter-sized forehead bruises that mark fervent praying. Though they don't always admit it when asked, many are members of the Muslim Brotherhood, the outlawed Islamist group that is Egypt's largest political organization.

Though the Brotherhood didn't participate in the initial demonstration on Jan. 25 organized by human-rights activists that sparked Egypt's democracy uprising, its role has been increasing ever since, especially in the front lines of the rock-throwing battles with government supporters. Ironically, the Egyptian government, which initially tried to discredit the protests by blaming them on the Brotherhood (a tactic the government has used often in the past to win foreign support for repressing dissent) is in part responsible for bolstering its position. Having faced government repression for years, the Brotherhood's devotees were among the best prepared for the wave of violence meted out by the government. "All the liberal and leftist groups aren't organized, but the Brotherhood is organized," says Malek Labib, an Egyptian doctoral student who had returned from university in France to spend several nights in the square during the worst of the attacks, bringing water and rocks to the frontline fighters. "They're not the majority, but they're the most courageous." (Read "Are Egypt's Anti-Mubarak Protesters Winning or Losing?")

As of Sunday, Feb. 6, the Brotherhood has made a political breakthrough as well. Departing from 30 years of official policy, the Egyptian government included the Brotherhood among the array of opposition groups invited to talk with Vice President Omar Suleiman, the Mubarak regime's new front man. After the talks ended without progress, some opposition groups muted their previous demand that President Mubarak leave immediately, but Brotherhood officials vowed to continue the protests until Mubarak resigns.

Still, the newfound prominence of the Brotherhood has added to the long-held concern among American policymakers and their Israeli allies that a collapse of the Mubarak regime could be the beginning of a slippery slope toward a Islamist takeover of Egypt  much like the Islamic Revolution in Iran or the Hamas takeover of Gaza  and the renunciation of the U.S.-brokered Camp David peace treaty between Israel and Egypt. Think tanks sympathetic to Israel and U.S. Jewish groups have begun circulating memos and talking points among opinion makers that cite some of the Islamist group's many incendiary anti-Israeli comments in recent years as well as examples of the Brotherhood's links to Hamas. "We will continue to raise the banner of jihad  two swords and a Koran  as long as the Zionists raise their flag, with two blue stripes to represent their so-called state," said Brotherhood leader Muhammad Badi in 2010, according to the Washington Institute of Near Eastern Affairs. (See President Obama's thoughts on the Muslim Brotherhood.)

But such language is at odds with how the Brotherhood and its supporters have behaved so far in Tahrir Square. From its battle-scarred frontline skirmishers to suited politicians, the Brotherhood is downplaying its role in the Egyptian uprising and downplaying the role of any religious or class animus. "[Critics] say that this is a revolution of the poor against the rich," called out a preacher during midday prayer services on Friday, Feb. 4. "They say this is a revolution of the hungry against the fed, of one sect against another. But it is not. It is a revolution for freedom that God ordained!" Finding a Brotherhood member who would brag about the group's exploits in battle with the police was difficult. "This is a people's revolution, and the Brotherhood is just a section of the people," says one supporter. Officials were clear about giving the young activists who started the rebellion credit where it was due. "This is a youth movement led by the youth," says one. "The Brotherhood is just a participant. I believe we are 5% to 10% of this, maximum."

Moreover, Muslim Brotherhood leaders interviewed by TIME in Tahrir Square consistently spoke of their commitment to the civil, nonsectarian nature of the state. "The Muslim Brotherhood takes Islam as a template, but we don't have a religious state or God-ordained rule," says Ibrahim Zakaria, a Brotherhood official and former Member of Parliament. "We believe in democracy and all its rules. We believe in the principle that the people are the origin and source of sovereignty and that the people choose their leaders in free and secret ballots."

On the subject of whether a new Egyptian government should cancel the Camp David Accords, they demurred. "We are not going to cancel any agreement previously made by the government," says Zakaria. "But if there is a referendum about this or any other agreement, then we obey the people's will." (See pictures of the confrontation on Tahrir Square.)

But by and large, the democracy movement in Tahrir Square may be transforming the politics of the Brotherhood  and of Egypt  by exposing them to the breadth of opinion and identity freely in the public realm for the first time. Tahrir Square buzzes not just with chants against the government but also with conversations among Egyptians of all types, in which everyone is entitled to opinions that they can finally air. "It was the government that created false enemies, because it had no legitimacy," says Mohammed Chalabi, an Arabic teacher. "When we are a free country, we won't need any enemies."

That very openness  if it continues despite a resurgence of Egypt's security state in recent days  may in the end reduce the role of the Brotherhood as the country's best-organized opposition once normal political parties are allowed to form. Anything can happen in a democracy.

"It's unclear who the opposition even is at the moment," says the Brotherhood's Zakaria. "We can't even define it. This is an illegal regime that made opposition illegal. So we are calling for free elections so we can find out just who the parties actually are."